Читать книгу The Homing Instinct: Meaning and Mystery in Animal Migration - Bernd Heinrich - Страница 10

THE TENT CATERPILLAR MOTH, MALACOSOMA AMERICANUM, is common in North America. It emerges from its light yellow silk cocoon in late summer, and the female is then ready to deposit her batch of over a hundred eggs. She searches for an apple or a cherry tree, and somewhere out on a thin twig of just the right diameter — about a half centimeter — she exudes her eggs along with sticky foam to form her egg mass into a ring that wraps around the twig. The foam dries and hardens, encasing the clutch of eggs and gluing them solidly to the branch where they stay through the coming winter. But the larvae develop inside the eggs during the summer and, while confined in their eggs through the winter, hatch at almost precisely the day, about nine months after the egg-laying, when their tree breaks its buds.

The moth is named for the conspicuous communal homes of silk, called “tents,” that its caterpillars make, and in the spring of 2013 I found a just-made tent on a young black cherry tree next to my Maine cabin. Like nearly everyone else in this part of the country, I was long familiar with these caterpillars but had not deemed them worthy of a closer look. The tents act, I learned, like miniature greenhouses and warm the new caterpillars at a time when nightly frosts are still common. But, despite its advantages, to have any home is to incur costs: it has to be made, and it takes time, energy, and expertise to make, and the wherewithal to travel to and from it. For the time being, I wanted to know where the caterpillars making this home had come from. To my surprise, the ring on the twig with the now-emptied eggs I was looking for was almost a meter from the tent. How had the many hatchling caterpillars “decided” or been able to stay together and then coordinate to make their tent? Squinting against the sun, I could see a glistening trail of fine silk leading from the emptied egg-case ring to their home, so here was at least a hint as to how they crawl together to end up at the same place.

On the second day after I found the tent, May 1, there was still snow on the ground in the woods. There was as yet no sign of fresh green anywhere. But I wrote in my journal, “Black cherry buds ready to pop leaves.” These trees are the first to leaf out, and the caterpillars could not have fed yet. What would they do? An hour after the sun came up, the tiny caterpillars emerged from their tent and massed on its sunny side. An hour later they started milling about, and then a few started crawling, seemingly aimlessly, several centimeters up and down the trunk and branches of the cherry tree.

As I had anticipated, some of the tiny caterpillars started to crawl back onto the same branch they had come from, possibly following their previously made silk trail. But they went only six centimeters before turning back. Others went down the trunk of the tree. Always some would turn back, and then the others followed one behind the other in a line. Finally, by 7:30 a.m., a contingent of about twenty of them had progressed nine centimeters down the tree trunk, although two were coming back up. Then more started to leave the tent, and eventually all were in one long line, going only down the trunk and then angling up another branch. In half an hour the leaders had traveled seventy-three centimeters and reached a bud. The rest were strung out all the way to the tent, but their two other travel-direction options had been abandoned. All were eventually massed at the same cherry bud, three-quarters of a meter from their tent, and in an hour and a half they had all returned to their tent, one following the other in a long train.

The young black cherry tree showing relative locations of a tent caterpillar moth egg cluster (C) from which the clutch of just-hatched caterpillars emerged and traveled to start making their home (H) in a crotch of the tree, and their first travels as a group (T) to feeding places

At noon they came out and crawled onto the outside of their tent, waving their heads back and forth, apparently weaving silk from their salivary glands to enlarge it. Another hour later they were again all massed inside the tent and perched, immobile, tightly against the bark, where they were barely visible through the thin gossamer veil of silk.

The caterpillars stayed in their tent through the night, and I expected them to go at sunup to the same branch where they had been the day before. But instead, this time they all followed an entirely different path, going directly up the tree instead of down as on the previous day, and without taking another side branch. I could not detect any silk on their so-far two different foraging trails, and this time they went even farther — a distance of 130 centimeters. After their one meal the day before, they were already noticeably larger. A few were the same size as the day before, but most had probably doubled in weight. There were many tiny fecal droplets in their web. So they had fed, even though it seemed hardly possible that they had anything to feed on at the barely opening bud.

On the third day the buds had opened and the tree was replete with new small leaves pushing out of the buds. But it had been a cool night — there was again frost on the ground at dawn — and the caterpillars made a slow start.

The pattern soon became clear: the caterpillars spent most of the night and most of the day when they were not feeding in their home. The time spent on tree branches was brief, and it could not have been just to keep warm that they stayed in their home because they went back inside just as quickly after feeding regardless of temperature or time of day.

Having found and watched the caterpillars of one tent, I then observed others for more clues to their homing behavior. One of the surprises to me was that as they grew larger, they foraged independently of one another, no longer going to and from feeding areas in groups. Furthermore, after they were about half grown they left their tents, not to return at all but still to continue feeding before eventually searching for a spot in which to spin their flimsy cocoon. Tent caterpillars usually choose a bark crevice to pupate, although commonly they also choose the cracks in the sides of buildings. But why were the young caterpillars strongly homebound and the older ones not?

I suspect the young ones’ web-making behavior may have evolved in part as an anti-predator response. The tents were visited by red wood ants, Formica rufa, and right after the caterpillars hatched, these ants often loitered alongside them on their trails. I tore a nest open on one side to find out if it served as protection. It must have, because ants entered, though frequently wiping their antennae as though irritated. Nevertheless they tarried inside the damaged nest, and I saw them grab and walk off with caterpillars. No ants entered an intact nest of the several I watched, each of which consisted of several successive layers of silk. Thus, the webbing of the tent acts as a deterrent to predators such as ants. Staying inside the home most of the day and night, as these caterpillars appear to do when they are small, probably reduces mortality from parasitic flies and ichneumon wasps as well. When they are larger, the caterpillars are probably protected from the ants, as well as from most birds, by a layer of fine spines. They pupate without having to bury themselves to escape frost, because the adult emerges long before there is any frost.

Because these caterpillars are protected from predators in the summer homes they build and by the spines they wear, because they mature early enough in the summer for the pupae to avoid the cold of winter (by early emergence of the moth), and because the eggs and young larvae are immune to freezing because of the antifreeze they contain, “everything” in the life of the tent caterpillar moth may be found within a few meters. The adults that emerge in late June are not far from the apple or cherry trees where the parent left her eggs, and their life cycle can be completed without their having to go far from home, unlike some other insects which traverse a continent to be able to satisfy all their needs.

Monarchs. Of all the insects, the travels of the monarch butterfly, Danaus plexippus, are perhaps most famously spectacular in both scale and scope. Dr. Lincoln P. Brower of the University of Florida in Gainesville (now at Greenbriar College), who has studied this butterfly and its migration for over forty years, records the rich history of the emergence of our knowledge of monarch migrations. Early naturalists saw “immense swarms” in the prairie states where the caterpillars fed on the leaves of the many native species of milkweed (Asclepias) and the adults fed on the nectar of their flowers. Monarchs declined when later industrial agriculture destroyed many of their food plants, but in the nineteenth century they resurged in the East due to land clearing and the spread mainly of one milkweed, A. syriaca. Millions of them were seen passing for hours, even in Boston. This was a phenomenon that is hard to imagine now and it ignited much interest then. Charles Valentine Riley, the entomologist who first hypothesized that these butterflies engaged in a birdlike migration, cites people seeing them in the fall in swarms that extended for kilometers and obscured the sun, “blurring day into night.” Huge lines of them passing Boston in 1880 were described as “almost beyond belief.” Now, with reforestation, plowing, and then the use of Roundup and other weed killers that eliminated their food plants in agricultural fields, the monarch is but a shadow of what it was. In the past several years in the East, it seems to have almost disappeared. For the first time, I saw not a single one in late summer of 2013. But our knowledge of the scope of the monarch migration has blossomed.

Monarchs migrate on their own power for thousands of kilometers, and, unlike many other insect migrants, the population (though not the individuals) has a regular two-way migration, although as with the other insect migrants, the individuals that come back are not the same ones that left.

Unlike most of the other North American butterflies and moths, which overwinter in New England as eggs, larvae, pupae, or adults, monarchs cannot survive there through the winter in any stage. The population that normally now graces fields all along eastern North America overwinters at around three thousand meters’ elevation in dense fir groves on the southwest slopes of volcanic mountains thousands of kilometers to the southwest, near Mexico City. The monarchs find shelter in those fir stands from rain, hail, and occasional snow. It is not cold enough for the butterflies to freeze there, but it is cool enough for them to conserve the energy resources that they have accumulated on their way south.

The monarch butterfly adult, caterpillar, and chrysalis

In the summer, the monarchs fly in what look like random zigzag patterns over the New England fields as they stop here and there to sip nectar. Occasionally you see a mated pair, the female doing the work of flying, the male dangling passively with folded wings while attached by his genitals. After the prolonged mating (and/or technically “mate guarding,” since it prevents mating by other males), the female glues her delicately patterned green eggs with gold markings, one at a time, to the undersides of milkweed plants. In a few days, the flashy yellow-black-white larvae hatch and start chomping. After about fifteen days (depending on the temperature), the caterpillars have increased their weight to 1.5 grams (2,780 times the hatchling weight). The caterpillar attaches itself to a support such as the underside of a leaf by a clasping organ at the hind end of its abdomen to hang upside down. It will then molt into the bright green pupa (chrysalis) with the shiny golden spots that is surely familiar to almost all school kids. In a few days, the chrysalis starts to turn dark, and the outlines of the orange-patterned wings are visible through the now-transparent cuticle. When the chrysalis splits, along a predetermined line of weakness in the back, the limp adult slips out and expands its wings, and in two or three hours hormones will have instigated a biochemical process that hardens its body armor and stiffens its wings. The butterfly is ready to fly. Where will its wings take it?

Thanks to the monarch studies initiated in 1935 by Dr. Fred A. Urquhart and his wife, Norah Urquhart, from the Zoology Department of the University of Toronto and continued to the present day with the input and cooperation of thousands of amateur volunteers, there is now an amazing story to tell. The Urquharts noted in the late 1930s that the monarchs they saw in late May and early June in Canada had tattered wings, and they knew that this species would not and could not overwinter in Canada, so they suspected that they may have come a very long way. Monarchs fly in a southwesterly direction in the fall, but nobody had a clue where they ended up. To get some idea of the butterflies’ movements, these researchers in 1937 began gluing paper tags onto monarch wings with this message: “Please send to Zoology University Toronto Canada.” Monarchs weigh almost half a gram and the wing tags only 0.01 gram, so the tags were not likely to hamper the animals’ movements. Similar tags, used today, have pressure-adhesive backing and can be folded in half and glued over the leading edge of the forewing (after the scales are removed).

The idea from the inception of the monarch-marking studies was to try to find out if the butterflies migrated — an idea that at the time, as Urquhart noted, “was considered quite impossible.” But the question of where the butterflies might be going to and coming from grabbed the imagination, and anyone seeing a tagged butterfly would be sure to try to catch it. Sure enough, tags were returned over decades that suggested a migratory pattern. Individual tags were returned from huge distances, up to 1,288 kilometers. One monarch that was tagged in Ontario in 1957 was recovered eighteen days later in Atlanta, Georgia, 1,184 air kilometers distant. Clearly, when the butterflies left Canada in the fall, they headed south.

Still, nobody knew what happened to the mass of butterflies. Then, in January 1975, Cathy and Ken Brugger of Mexico City found them — a dazzling, shimmering, orange display of an estimated 22.5 million monarch butterflies on one 2.2-hectare site (which turned out to be only one of ultimately thirteen overwintering sites in the mountains of Mexico). The millions of monarchs were festooned in the trees in the mountains of Michoacán near Mexico City. The Urquharts excitedly traveled to see the site and on January 18, 1976, listened to “the sound of the fluttering of thousands of wings [that was] like that of a distant waterfall.” As they stood awestruck by this dazzling display, a pine branch broke off from the sheer weight of butterflies attached to it, and it crashed to the ground right in front of them. Fred Urquhart had been posing for a National Geographic photographer surrounded by these just-fallen butterflies when, incredibly, he saw a tagged one among them. When he traced its origin, he learned that it had been tagged on September 6, 1975, by Jim Gilbert, from Chaska, Minnesota. Urquhart, who had encountered countless tagged butterflies in his career, said it was “the most exciting one I have ever experienced.”

The picture that has now emerged from decades of study is that individual butterflies migrate all the way from Ontario to Mexico in the fall, arriving there at their overwintering sites in a torrent during October. They spend most of the winter in Mexico in a cooled low-energy state but soar around on warm days to drink water and replenish on nectar. In early spring, when their sex urge awakens, there is a mating orgy followed by a mass exodus. Most of the females mate before leaving, and their “compasses,” which were set to take them south in late fall, are now “reset” to take them in a northerly direction.

As the tide of butterflies advances northward, the females stop to lay their eggs on milkweed. Some of the butterflies from Mexico make it all the way to the north, and others (their offspring) that grow from the eggs laid along the way arrive later. Those of the first generation have slightly tattered wings when they arrive in the north, while those that arrive later have untattered wings. (However, not all monarch populations migrate, and not all that do, travel in the same directions as the populations of northeastern North America.)

One of the mysteries that puzzled Fred Urquhart was how the butterflies home. In Urquhart’s 1987 book on the monarch, he speculated that the butterflies perhaps use the Earth’s magnetic lines of force, although different populations of the butterfly migrate in different directions, so they could not all be orienting to it in the same way.

A potentially even more puzzling question is the ultimate (evolutionary) one of why these butterflies migrate in the first place. Urquhart simply suggested what he admitted was a “perhaps far-fetched” idea: that “twice each year it [Earth] passes through an area rich in some sort of radiation that could impinge upon animal life [that] might affect in some manner the cells of the body causing reproductive organs to abort in the fall and develop in the spring and initiate the migratory response.” This is an unlikely theory, though, mostly because it depends on a mechanism that is not adaptive in evolutionary terms. Instead, more current thinking about the adaptive reason why the phenomenon has evolved focuses on energy economy and maximization of resource use under the expected evolutionary constraints from the monarch’s having evolved in the tropics, meaning it was not able to survive northern winters. (Monarchs belong to the family Danaidae, an otherwise strictly tropical group.) Migration to the north in the spring opens up the milkweed crop over a major swath of North America as a food base for the larvae. In addition, the journey is probably not costly to the monarchs, either in terms of predation (since they are chemically protected from predation by poisons they sequester from their food plants) or in terms of energy costs, since their energy intake along the way more than makes up for the energy expended for travel. Indeed, unlike most birds that may deplete all their fat reserves on migration, these butterflies instead fatten up on their journey and may consist of about 50 percent body fat by the time they arrive in Mexico, where their overwintering fast begins.

Butterflies and moths experience tremendous selective pressure, and undoubtedly there are constant readjustments of survival strategies. Weather affects the populations, not only through flight activity and flight range as well as growth rates of larvae, but perhaps also indirectly by influencing virus infections. But Urquhart noted that each female monarch butterfly lays up to seven hundred eggs, and he calculated that the “biotic potential” — the number of individuals if there are no deaths — of one female after only four generations (that is, at the end of one summer) is 30,012,500,000 adults. Luckily for the planet, animals’ reproductive potentials are never naturally realized, for long. The limit is quickly reached when the population uses up its food base, in this case milkweed. In some years a virus decimates most of the monarch population over North America, but then several years later it rebounds. But the population cannot rebound from some things: in recent years there have been massive declines of the monarch population that cannot be reversed, because they are due to unnatural causes — the massive conversion of land to crops, and the introduction of genetically modified crops that tolerate herbicides, which have allowed the elimination of milkweed that formerly grew between rows of corn.

The flight performance of monarchs is spectacular, but like the hordes of cluster flies from the surrounding fields and woods that overwinter in my cabin, they are traveling to a specific place for overwintering where they have never been before. Such homing movements are diverse, but common. Robert D. Stevenson and William A. Haber of the University of Massachusetts, Boston, found a regular seasonal migration of about eighty percent (250 species) of butterflies living in the dry lowlands of the Pacific Slope of Costa Rica that migrate to wetter forests of the east. Distances traveled range from ten to a hundred kilometers.

In North America as well as in Europe, the cosmopolitan painted lady, Vanessa cardui, a mostly orange and black butterfly with white spots and pink and blue “eyes” on its under-wings, at times appears in large numbers and then is not seen again for years. Usually the individuals are seen crossing a road, and almost all will be heading in the same direction. The painted lady regularly migrates north from Mexico, from where it originates, after heavy rains in the deserts have created an abundance of food plants, primarily thistles. A friend told me of one migration while he was in Arizona when his windshield wipers “soon became useless” because of the huge numbers of painted ladies plastered onto them as he was driving. I see them regularly in Vermont and Maine, but seldom in large numbers (the summer of 2012 was one of the exceptions).

Red admiral butterfly larva, adult, and chrysalis. The larva makes a shelter for itself by pulling leaves together and holding them with silk, while then feeding on the leaf.

One of the butterflies that not only migrates as an adult but also hibernates in some parts of its range is the red admiral, Vanessa atalanta. It is (as are all butterflies!) beautifully colored. It sports a wide red stripe across each dark forewing ornamented with white spots, and its larvae feed on nettles. I wrote in my journal on May 11, 1985, near my home in Vermont: “In the afternoon from around 2:30 to 4:30 PM, as I was jogging along on an 18-mile circular loop I counted 512 red admiral, crossing the road in front of me. All but 5 of these were flying in a northeasterly direction. At 5:00 PM, after I was home, I take compass readings of butterflies flying over a plowed field where they funnel onto it through a valley. I can see them to take a bearing for at least 50 paces — 250 feet. All 22 that I observed flew in NE direction. At 6:00 PM activity almost stopped. The breeze is slight, from northwest.” In the summer of 2001 and again in the spring of 2010 I saw large numbers of red admirals. They fed on freshly opened apple blossoms, and later all the nettle plants in a neighbors’ sheep pasture had an abundance of their caterpillars.

Moth migrations are perhaps more spectacular than those of butterflies. Jason W. Chapman and colleagues report one recent ten-year study involving radar tracking of about one hundred thousand owlet (Noctuid) moths, primarily the silver Y moth, Autographa gamma, migrating south in the fall from northern Europe, and then north from the Mediterranean in the spring. Like the butterflies, these insects breed along their migration route. Also like the butterflies, the moths partially correct for crosswinds, to maintain specific directions. Most surprising perhaps is the moths’ windsurfing; they choose the most favorable wind currents corresponding to their respective spring or fall migratory directions. If the wind shifts about twenty degrees from the favorable direction, they adjust their flight to accommodate and maintain the correct direction. If the wind shifts ninety degrees, though, they stop and wait for a favorable wind. Millions of them fly together in the dark of night, and, like the monarchs’, their compass directions are likely tuned to the Earth’s magnetic fields. Some studies of radio-tagged green darner dragonflies, Anax junius, suggest that these insects also migrate hundreds to thousands of kilometers from north to south with those that return being a different generation.

These behaviors get the animals to a good place (for overwintering or for reproduction). Like the long-range movements with specific endpoints on the map, homing to a good place is not always easily distinguished from moving out of a bad place. The behavior is a mechanism with deep evolutionary roots. Indeed, insect wings (and metamorphosis) may themselves have been an original adaptation for dispersal, to colonize temporary pools, animal carcasses, or other temporary resources. The first individuals to reach the resource won the competition to use it and multiply there, and these were more likely to be the ones that flew, and flew far and wide, rather than those that walked at random.

Wings and metamorphosis have lesser value in constant conditions. Some insects are able to respond in real time to the changes in conditions they experience (especially crowding), in that when they don’t “need” to disperse they either don’t grow wings (some aphids) or the muscles to power the wings are broken down and the amino acids from the protein are used instead to make more eggs (in some Hemiptera bugs). Often there are discrete “dispersers” versus “non-dispersers” in any given insect population, and the percentage of each depends on the quality of the home habitat and hence the relative cost/benefit ratio of moving versus staying.

Dispersing to “anywhere but here” generally applies to nonmigratory species that have no encoded or learned directions to go to but may have innate instructions to move in more-or-less straight lines rather than potentially going in circles in order to achieve distance. In Africa, dung-ball-rolling scarab beetles race away from their often thousands of competitors at a dung pile at night by using the swath of stars of the Milky Way galaxy as a reference. Swarms of insects feeding at dung and carcasses also attract predators, and as soon as they finish feeding, many distance themselves from those predators. I’ve observed blowfly larvae at animal carcasses keeping to almost perfectly straight lines in their getaway at dawn, by steering directly toward the direction of the rising sun. Mass movements sometimes observed in some rodents, such as lemmings and gray squirrels (as in 1935 in New England) following a population explosion after a superabundance of food, may be another example of dispersal to get to a better place, though not necessarily a predetermined one.

On the other hand, “true” migrants are able to utilize ideal conditions in two places, provided they vary predictably. Arctic terns, Sterna paradisaea, breed throughout the Arctic, then fly to Antarctica to escape winter when food availability declines and to arrive in spring and food again, a round-trip distance of nearly seventy-one thousand kilometers. Gray whales, Eschrichtius robustus, also feed in the Arctic in the summer but then travel eight thousand kilometers along the coastline to Mexico to bear their calves in warm waters.

Dispersers are not neatly differentiated from migrants, although the first commonly rely on passive mechanisms as opposed to the migrants, which move to specific goals by their own powers of locomotion. There are all gradations in between. Each case is unique, and there are thousands. Let’s look at more ways of getting to a good place, as represented by eels, a grasshopper, aphids, and ladybird beetles.

Eels. There are many species of eels, but the American, Anguilla rostrata, and the European, A. anguilla, the species with which we are most familiar, live most of their lives in freshwater ponds and lakes. For reasons that are not clear, though, they do not reproduce in their home areas. To the contrary, both disperse (or “migrate”) thousands of kilometers on a one-way trip to spawn and then die in the mid-Atlantic.

Just as birds and some insects use air currents, eels use water currents to help them leave their lifetime homes. After eels leave their freshwater homes and head for the ocean to spawn and die, their larvae then drift in the ocean currents for years. But eels’ dispersal behavior is anything but passive. Eels fatten up to prepare to leave their home haunts in the bottoms of freshwater lakes and streams. Before they head for the sea, they absorb their digestive tracts and transform themselves by greatly enlarging their eyes and turning silvery on their ventral side. The latter transformation produces counter-shading that reduces their visibility to predators below them in the open ocean waters.

The eels’ dramatic changes in behavior, morphology, and physiology that enable them to switch from living in freshwater habitat to open ocean highlight the operation of strong selective pressures. But why do they leave their homes in freshwater ponds where they grew up and lived most of their lives? Their one-way, once-in-a-lifetime migrations to their Sargasso Sea spawning grounds can’t be to find a better feeding ground, or to escape competition. However, I suspect what the behavior accomplishes superbly is that the adults, which are predators, do not come in contact with and feed on their own young. Is it a mechanism that has evolved because it reduced predation on themselves?

There are over six thousand publications on eels, but the life cycle of these economically important food fish is still murky and has a long history of speculation. For centuries, nobody ever saw a baby eel, and even now, their spawning has not been witnessed. Aristotle presumed that eels grew from earthworms. The first young of eels, transparent leaflike forms, were found in the open Atlantic Ocean. Gradually, as more of these leaflike creatures were collected, it was noted that they varied in size, and that the smallest ones were found south of Bermuda in the Sargasso Sea, which was therefore presumed to be the site of their origin, that is, the eel spawning area.

The eel larvae, after hatching in the waters of the Gulf Stream, drift north. Like plankton, they move at the whim of the prevailing oceanic current. As they grow from about five to six centimeters in a year, they take on a more eel-like form but remain transparent. They are by then able to swim and, presumably by scent, find and swim up a river. Unlike salmon, however, these transparent “glass eels,” as they are known at this larval stage, can have no specific home stream scent to follow, because they have experienced only the scents of the ocean.

The female glass eels at this stage, in early spring, migrate up rivers and streams along the East Coast of America. After two months in a river they grow to about ten centimeters. Now known as “elvers,” they are no longer transparent, and they enter lakes and become eels. These female eels live in lakes for eight years or more, fattening up (the males stay in the saline estuaries). When they achieve the right amount of fat, these females become sexually mature. Each develops a clutch of three to six million eggs, and then one fall the gravid females start their journey downstream back to the ocean, to the Sargasso Sea to spawn. Since the males don’t live in fresh water, somewhere in the ocean the females then apparently meet the males for fertilization.

As the Gulf Stream continues north beyond North America, the larvae of the European eel, which originate from the same apparent spawning area, the Sargasso Sea, as the American eels, continue their journey. Finally, in two or three years, they reach the coasts of Europe, where they then also seek rivers and streams. Migrating upstream, they become pigmented, and after growing to adulthood, they migrate back into the open Atlantic and make their sixty-five-hundred-kilometer return to the Sargasso Sea, where they spawn on average several million eggs, and then die. Only one of several million of them will make the return journey to grow to a reproducing adult in fresh water.

Grasshoppers. One of the best-known insect dispersers, the “migratory locust” (the grasshopper, Schistocerca gregaria), engages in some of the most spectacular mass movements in the animal kingdom. On the African continent, this species has been famous since biblical times. Swarms of the “locust” have blackened the skies, and as those in the vanguard settle onto the earth and consume every green thing where they land, the rest fly over them until they reach more green, while those behind then take flight and do the same, and so a horde of hundreds of millions moves along, stripping all vegetation in its path. Predators cannot put a dent in those hordes. Additionally, the migratory locust is distasteful to potential predators because when it migrates, it is not choosy about what it ingests and takes up toxins from poisonous plants, which it incorporates into its tissues. The grasshoppers’ bright red-orange and yellow coloration, like that of many insects including the monarch butterfly, reminds potential predators of its distastefulness.

Nymphs and adults of the two phases of the migratory grasshopper

Although this distinctively colored grasshopper appears to arrive suddenly, it is often there all along, but in a different guise. It has a green solitary grass-fed form that blends in with its food and that is palatable to predators. For a long time scientists thought that the grasshopper “migrants” that appeared so suddenly were a unique species, one arriving from an unknown origin and heading for an unknown destination. Now we know that the migrants are a “phase” of a common species that changes its color, form, and behavior in response to crowding. Proof comes from experiments: to create these “migrants” from isolated individuals one takes a nymph (immature stage), puts it in a jar, and has a motor-driven brush tickle it continuously. The constant tickling mimics the crowding, which in the case of S. gregaria is the signal evolution has “chosen” to trigger the nervous system to alter the hormones that result in development into the restless migratory phase of a different color, wing length, and behavior. It is a good example that shows environment is “everything,” or from another perspective, it’s all about genetics.

Migratory-phase locusts are highly irritable and will jump up and follow a crowd flying over it. This behavior removes the grasshoppers from an area that is overpopulated and brings them to new land where conditions are conducive to feeding, egg-laying, and growth of their offspring. Although the grasshoppers could have no knowledge of where such a distant but good place might be, they migrate to it as if they do.

The grasshoppers reach a consensus. It is a sensible one, although it involves no thinking and no discussion. They simply fly up to join the crowd, which follows the prevailing winds. Eventually these winds meet air from an opposite direction and, when moist tropical air rises into cooler altitudes, rain precipitates out of the resulting clouds, depositing the grasshoppers to earth along with the rain. As the ground is watered and softened, the grasshoppers can shove their abdomens into the soil to lay their eggs. The new nymphs hatch just as new food starts to sprout. Their homing (or “dispersal”?), which has ended at this good place for them to reproduce, is now complete.

Aphids live in crowded “colonies” on plants into which they insert their mouthparts, much as mosquitoes puncture skin, except that they imbibe plant sap instead of blood and may stay plugged in at the same spot for most of their life spans. One might suppose they could not or would not migrate. But, like the migratory grasshoppers, they may travel possibly hundreds of kilometers. Nobody knows for sure how far; it depends on the prevailing winds.

Sedentary aphids already on good food do not leave to seek, or even require, mates. Instead, they switch to virgin births after a sexual migratory phase. Daughters then settle directly next to mother, and so on and on for many generations as the colony grows. And then, cued by the shortening of the days in the late summer and fall when the food supply runs out, the aphids’ offspring take a different developmental route. Because of changing day length and/or food, the nymphs on their final molt grow wings and become sexual. Frail and weak-winged they are, but an aphid is light and carried by the wind much like the seed of a dandelion or poplar tree, or a baby spider on a thread of silk. I usually see them in September when they appear like flecks of white lint floating erratically in the air.

To reach wind the aphids fly or are wafted up. Eventually, they don’t fight the wind but drift along and settle somewhere back down to earth. On their descent, assisted by their wings, they head toward anything colored light green. This color (unless they are tricked by pieces of green paper coated with sticky glue left by an insect physiologist studying them) is likely to be associated with their favorite food, fresh plant growth. After landing, perhaps because the chances of a mate arriving at precisely this one tiny spot of residence are remote, they switch back to virgin births and thus restart the cycle.

Ladybird beetles, the predators of aphids, similarly have adapted by migrating in a seasonal environment. In the western United States they migrate mainly on their own power from lowlands up into the Sierras, where they can in some locations be scooped up by the bucketful (generally to be sold to farmers and gardeners — to control aphids!). At the campus of the University of California at Berkeley, I often saw streams of them flying or being blown uphill in Strawberry Canyon by the campus when the grass was drying after the spring rains.

Ladybird beetles of some species migrate when reproduction must cease for the season. Despite the energy they expend for flight, they may migrate largely to save energy. It goes this way: As long as there are plenty of aphids to be had, both larvae and adult ladybirds don’t go hungry. Eventually, however, the green vegetation suitable for aphids disappears in the hot California summer, and so the aphids leave. Now the beetles’ resting metabolism kicks in as a significant liability. Resting metabolism for beetles is high at high body temperature but becomes almost negligible when they are torpid at the lowest body temperature tolerated, near or slightly below freezing. An elevated resting metabolism, month after month in the western states’ dry hot summer, would deplete both the beetles’ energy reserves and their body water. Without replenishment of food and water in that environment they would die. But by flying, with the aid of thermals, they are brought into upwelling air currents in the hills and then into the mountains where they reach cooler air. At this point, though, they do something different from the aphids: instead of being attracted to green, they are attracted to either red and/or the scent of each other. How else to explain that they crowd together into large groups in which they then overwinter? The advantage of their grouping behavior is not clear, but I suspect that it amplifies their noxiousness. (This is based on experience: ladybird beetles regularly come into my cabin to overwinter and quite often crawl into bed with me. I can vouch for the fact that they are noxious if not obnoxious, and several more so than one.)

The ladybird beetles arrive at a suitable place — a cool one — where they conserve their limited energy reserves during winter. The hypothesis that they home not just to an area, but also to a specific spot, is based on observations that my friend and colleague Dr. Timothy Otter has made in the Sawtooth Mountains near Stanley in Idaho. The beetles there were known by local ranchers to aggregate every fall in large numbers in specific rock cairns of decomposing granite in the hills above the valley floor. Otter, a biologist, wondered why the beetles aggregated there, in specific spots, and not in similar places nearby.

Since hibernation concerns adaptations related to temperature, Otter concentrated his efforts on unraveling the beetles’ temperature tolerance and compared it to the temperature at that site. But, curiously, there was nothing unique about the temperature of the specific site on “Ladybug Hill,” relative to other sites near it. Nor did the grouping of the beetles affect their temperature; the temperature of their aggregation was indistinguishable from ambient temperature. So, they didn’t aggregate to keep warmer than the environment there.

As already mentioned, the one thing all ladybug beetles do have in common is that they stink. The evolutionary significance of their aggregations is therefore likely the conventional one of other animals, namely, to advertise their noxiousness for protection from predators. Shrews and other predators may kill two or three victims and then spit them out, but they then learn to avoid them and so they do not continue eating the hundreds of thousands they would consume if they had found a bonanza of ladybirds. Given their safety of numbers, it is advantageous for any one beetle to join a group rather than to overwinter alone, because the chances that it will become a victim of a shrew’s educational process are reduced in more or less direct proportion to the group number. This rationale for large numbers of beetles, but of different individuals, massing repeatedly at the same location is not proven, but my colleague Daniel F. Vogt and I found that it applied to another noxious-smelling beetle, the whirligig (Gyrinidae) water beetles in Lake Itasca in Minnesota. These beetles there homed in on any existing groups of tens of thousands at dawn, after a night of foraging far and wide on the surface of the lake.

The second question is: How is an aggregation formed?

Insects have an impressive ability to home in on scent, and ladybird beetles could find the aggregation by following an odor plume. Memory cannot be involved in the ladybugs that Otter found year after year aggregating at the same site, which were generations removed from those of the previous year. The beetles arrive on site in September, stay there eight months, and in May return to the valley floor to feed, mate, and reproduce. Only their descendants, two or three generations later, could return to the tiny spot where they had hibernated.

The aphids’ rule of flying up toward the light, to then be dispersed by the wind, and then homing in on green when they settle could be a model of what happens in ladybird aggregation. Ladybirds are much stronger fliers than aphids, although they too are swept along in updrafts. But such drifting, while helping to account for their annual ascent from the lowlands into the mountains, does nothing to explain how tens of thousands of them end up under the same rock pile.

If the ladybirds home in on color, this could be tested, as the aphids’ homing in on food plants was tested — by leaving color targets at sites other than the traditional hibernaculum. But even if red color is an attractant (highly unlikely because the beetles aggregate under the rocks, not on them), that still would not explain their annual return to the same place in successive years. Do they smell their ancestors? Could thousands of smelly beetles piled up for eight months leave sufficient scent residue to serve as a marker that allows others to home in on the spot? If so, this idea could also be tested, by transferring an aggregation of beetles to overwinter at another physically similar place in the same general area as the old to see if a new traditional homing site for their mutual protection is created.